by Lee Billings
I asked what all that meant for the later terms of his equation.
“We’ve found a truly great number of potentially habitable places, but the number of places where you could expect to find intelligent, technological life really hasn’t increased that much,” Drake replied. “That suggests to me there are probably significant barriers to the development of widespread, powerful technology. To surpass them, you might need a planet quite a lot like Earth. That may sound discouraging, until you realize just how many stars there are. Their sheer number suggests the equivalent of Earth and its life has probably happened many times before and will occur many, many times again. They’re out there.”
He chuckled, coughed, and creakily unfolded himself from the couch, clearly weary of sitting. We went outside to breathe fresh air.
Afternoon sunlight warmed our faces, and a cool breeze sighed through the towering redwoods to tousle Drake’s silver hair. The air smelled of green, growing things. Drake pointed out the Moon’s thin waxing crescent, faintly visible high in the cloudless sky. It was adjacent to the passing silver needle of a high-flying passenger jet. As we walked down into the yard, I gingerly stepped over the pale blue remnants of a robin’s egg cracked open on the front steps, fallen from a nest in an overhanging tree. The tide was rolling in far below us, down past the forested hills and beachfront suburbs, and surfers rode big waves toward the shore of Monterey Bay.
The scene from Drake’s front door encapsulated many of the essential facts of life on Earth. Fueled by raw sunlight, plants broke the chemical bonds of water and carbon dioxide, spinning together sugars and other hydrocarbons from the hydrogen and carbon and venting oxygen into the air. Sunlight scattering off all those airborne oxygen molecules made the sky appear blue. Animals breathed the oxygen and nourished their bodies with the hydrocarbons, utterly dependent upon these photosynthetic gifts from the plants. In death, plants and animals alike gave their Sun-spun carbon back to the Earth, where tremendous heat, pressure, and time transformed it into coal, oil, and natural gas. Mechanically extracted from the planet’s crust and burned in engines, generators, and furnaces, that fossilized energy powered most of humanity’s technological dominion over the globe. Built up and locked away for hundreds of millions of years, the carbon stockpile was gushing back into the planet’s atmosphere in a geological instant.
Our experience at Monterey Bay was a product of our planet’s physical characteristics—and the unlikely events that led to them. Earth’s abnormally large Moon, which stabilizes our planet’s axial tilt and bestows it with tides, was born when a Mars-size body collided with the proto-Earth early in our solar system’s history. Another impactor, a six-mile-wide asteroid, struck the Earth 66 million years ago and sparked a global mass extinction, ending the age of dinosaurs. Humanity’s small mammalian ancestors began their slow progress toward biospheric dominance, and the saurians that didn’t die out gradually gave rise to birds. Billions of years before the dinosaurs, the life-giving liquid we recognize as Earth’s ocean was mostly delivered by impactors, too, in a shower of water-rich asteroids and comets from the outer solar system. Earth’s aquatic abundance, it is thought, lubricates the planet’s fractured crustal plates and allows them to drift and slide in the geological process we call plate tectonics, a climate-regulating mechanism unique to our world out of all those in the solar system.
Turning away from the bay, Drake walked over to the center of his driveway, where the weathered stump of a giant redwood rose like a long-extinct volcano. He stooped and placed his hands upon the ancient wood. Years ago, he said, he had spread a thin layer of chalk on a section of the stump’s surface, allowing the growth rings to be easily seen, and set his young children to the task of counting them as an informal science project. They counted more than 2,000, one for each year of the tree’s life, which apparently began around the time of the birth of Jesus Christ.
“This tree saw the first light from the supernova that made the Crab Nebula, right about here,” Drake said, touching a point midway between the stump’s center and perimeter. Light from the supernova washed over the Earth in 1054, just as Western Europe was emerging from its Dark Ages. Sweeping his hand halfway farther out toward the perimeter, he brushed over the Age of Discovery, past rings recording the years when Europeans first explored and colonized the Americas. His hand kept moving until it slid from the stump’s edge.
Over the course of the tree’s 2,000-year existence, the Milky Way had fallen nearly five trillion miles closer to its nearest neighboring spiral galaxy, Andromeda, yet the distance between the two galaxies remained so great that a collision would not occur until perhaps 3 billion years in the future. In 2,000 years, the Sun had scarcely budged in its 250-million-year orbit about the galactic center, and, considering its life span of billions of years, hadn’t aged a day. Since their formation 4.6 billion years ago, our Sun and its planets have made perhaps eighteen galactic orbits—our solar system is eighteen “galactic years” old. When it was seventeen, redwood trees did not yet exist on Earth. When it was sixteen, simple organisms were taking their first tentative excursions from the sea to colonize the land. In fact, fossil evidence testified that for about fifteen of its eighteen galactic years, our planet had played host to little more than unicellular microbes and multicellular bacterial colonies, and was utterly devoid of anything so complicated as grass, trees, or animals, let alone beings capable of solving differential equations, building rockets, painting landscapes, writing symphonies, or feeling love.
By its twenty-second galactic birthday, some thousand million years hence, our planet may well return to its former barren state. Astrophysical and climatological models suggest that by then the Sun, steadily brightening as it ages, should increase in luminosity by about 10 percent—a seemingly minor change, but enough to render Earth’s climate too hot and its atmosphere too anemic to support complex multicellular life. Around that time, the oceans will begin evaporating, and most of Earth’s water will rapidly cook off into space. The loss of oceans a billion years from now marks the most likely expiration date for all life on Earth’s surface, though the omnipresent microbial biosphere might endure for billions of years further, shielded deep within the planet’s parched crust. Somewhere in the neighborhood of five billion years from now, the Sun will exhaust its supply of hydrogen and begin fusing its more energy-rich helium, gradually ballooning 250 times its current size to become a red giant star. Astronomers debate whether the Earth will be submerged within the hot outer layers of the swollen red Sun or whether it will escape relatively unscathed and only suffer its crust being melted back to magma. Either way, at that late date the life of our planet will be brought to a decisive conclusion.
Considering the long concatenation of astrophysical events that led to our habitable planet, and the unknown synergies of technology and geology that could shape its fate, the distinction between chance and necessity blurs. Given a few hundred million years, would life arise on any rocky, wet, warm world? Would intelligence and technology emerge only on worlds with histories that mirrored our own, replete with the equivalents of Earth’s Moon, mobile crust, and blue sky? Or was a focus on these features merely a failure of our Earth-bound imaginations? Was our planet and its history a useful template or a stumbling block in the search for alien life and intelligence? Would we even recognize our own planet as “Earth-like” if we glimpsed it a half billion years in its past or in its future? Answers to questions like these would be elusive as long as scientists only had one living world to study—our own. Drake didn’t believe they would remain intractable forever.
• • •
Back in 1960, I thought that the possibility of detecting extrasolar planets in my lifetime was very, very low, though Otto Struve had already given us ideas about how it might someday be done,” Drake had told me back in his living room. “I thought our only hope of detecting evidence of other planets was to receive radio signals from any intelligent creatures on them. We’re seeing a similar p
essimism play out now with characterization of planets around other stars. The techniques are there before us.”
Already, planet hunters had found a handful of worlds that in their most basic details didn’t appear too dissimilar from Earth. Those planets, their numbers growing every year, could potentially be much like our own. But the methods used to find them relied on closely observing a planet’s bright, beacon-like star, not the dim planet itself; the gravitational pull of a planet on its star, or the shadow a planet cast toward Earth as it transited across its star’s face, generally only revealed such things as a world’s mass, size, and orbital properties. Without actually seeing these worlds—that is, collecting and analyzing photons reflected off their atmospheres and surfaces—scientists would be unable to determine whether any potentially habitable, potentially Earth-like planet was actually either of those things. They would be stuck where Drake had been fifty years before, hoping against all odds for a message from the stars to come streaming from the sky, filled with information on the flora, fauna, and environment of a place far, far away.
During the nineteenth century, a series of incremental discoveries led to the breakthrough that enabled the bulk of modern astronomy: light emitted, absorbed, or reflected by matter changes its colors in a way that captures the matter’s chemical signature. Splitting up light into a spectrum to reveal those colors—a technique called spectroscopy—reveals those signatures, allowing astronomers to remotely measure the chemical composition of galaxies, stars, and planets. If they could somehow take a promising exoplanet’s picture by gathering enough of its reflected photons, researchers could use the resulting spectrum to investigate that world’s atmospheric chemistry. They could search for indicators of habitability, such as water vapor and carbon dioxide, as well as signs of life, like the free oxygen that filled and tinted our own planet’s skies. They could look for the glint of a parent star’s light shining off the smooth, flat surface of a planet’s oceans or seas, or even subtle changes in the color of land that would hint at photosynthetic plants. Astronomers using observations from satellites and interplanetary spacecraft had already performed all these measurements for the Earth, confirming that our living planet could, in theory, be studied from across the vast distances of interstellar space. Even if any extraterrestrials didn’t advertise their presence to the universe at large, techniques like spectroscopy offered hope that we could still find and study their home worlds.
In the last decades of the twentieth century, as exoplanetology became a legitimate scientific field, planet hunters devised several ways to take planetary snapshots across the light-years. All involved one or more custom-built space telescopes designed to nullify a target star’s glare and reveal its retinue of planets. At a likely cost of several billion dollars, a single space telescope could be built capable of delivering images of worlds around nearby stars, each planet manifesting as a dot a few pixels wide—minuscule, but more than enough for atmospheric spectroscopy. If money were no object, a fleet of telescopes could be assembled in space or on the far side of the Moon to act as one giant instrument, yielding larger images of nearby exoplanets that, though still very low-resolution, could reveal a world’s shorelines, continents, and cloud patterns. Such telescopes would go a long way toward determining whether a planet was worthy of being anointed “Earth-like.” Based on a fragmented astronomical community, an apathetic public, a gridlocked political system, and a struggling global economy, however, none appeared likely to be built anytime soon—at least not by the federal government of the United States of America.
Drake felt that if something could happen, somewhere it would happen, even if not right here and now. He wondered whether, if advanced cultures existed around nearby stars, they might have been watching our planet for quite some time using large space telescopes of their own.
“I’m speculating far out on a limb here,” he said as we walked around his yard. “But I would guess that most every civilization with technological capabilities slightly beyond our own uses lenses on the order of a million kilometers in diameter to explore the universe and communicate between stars.”
Beginning in the late 1980s, Drake had begun exploring an idea that made a lunar far side dotted with telescopes seem like child’s play. In retirement, the work had come to consume him, and now occupied much of his remaining time. He wanted to create a telescope that would surpass all others, one with a magnifying lens nearly a million and a half kilometers in diameter. Drake had found a way to transform the Sun itself into the ultimate telescope.
A consequence of the Sun’s immense mass is that it acts as a star-size “gravitational lens,” bending and amplifying light that grazes its surface. This effect, first measured during a solar eclipse in 1919 by the astronomer Arthur Eddington, was one of the key pieces of evidence that validated Einstein’s theory of general relativity. Simple math and physics, judiciously applied, show that our star bends light into a narrow beam aligned with the center of the Sun and the center of any far-distant light source. As first calculated by the Stanford radio astronomer Von Eshleman in 1979, the beam comes into focus at a point beginning some 82 billion kilometers (51 billion miles) away from the Sun, nearly fourteen times farther out than the orbit of Pluto, and extends outward into infinity. There are as many focal points and Sun-magnified beams as there are luminous objects in the sky—imagine a great sphere surrounding our star, its surface painted with amplified, high-resolution projected images of the heavens.
Reviewing Eshleman’s calculation, Drake had discovered that, due to electromagnetic interference generated by ionized gas in the Sun’s outer layers, ideal seeing conditions for this ultimate telescope weren’t at 82 billion kilometers, but almost twice as far out, at a distance of 150 billion kilometers (93 billion miles), a thousand times our distance from the Sun. For perspective, in June of 2011, humanity’s fastest and most-distant emissary, the Voyager 1 spacecraft launched in 1977, was just under 18 billion kilometers from the Sun, a bit more than a tenth of the distance to Drake’s ideal focus. It had taken thirty-five years to get that far from Earth. Clearly, utilizing our solar system’s ultimate telescope was a goal that could potentially take centuries to achieve. But the payoff might be worthwhile. Placed at any distant object’s given focal point, a light-gathering telescope on the order of 10 meters (33 feet) in size could beam images back to Earth about a million times higher in resolution than what a network of large telescopes on the lunar far side could deliver. If, for instance, we wished to examine a potentially habitable planet orbiting one of the two Sun-like stars in Alpha Centauri, the Sun’s nearest neighboring stellar system, a 10-meter telescope aligned with the Sun–Alpha Centauri gravitational focus could resolve surface features such as rivers, forests, and city lights. Put another way, a gravitational lens at Alpha Centauri could easily see the coastline of Monterey Bay, its tree-covered hills, and the bright lights of nearby big cities like San Francisco and Los Angeles.
“One of the beauties of gravitational lenses is that since the lensing object bends space, all light traveling through is equally affected,” Drake said, squinting into the sunlight beneath one of his lemon trees. “Gravitational lenses are achromatic—they work the same for optical light, infrared, everything. I like to think of what they could do for radio. If you had two civilizations around different stars in communication and aware of each other, they could use gravitational lensing to set up transmission and receiving stations on each end. You look at the numbers, and at first it seems totally insane, but this is real. You could transmit, let’s see, high-bandwidth signals from here to Alpha Centauri using only one watt of power. . . .”
He looked at me expectantly, but I could think of nothing to say.
“That’s the transmitting power of a cell phone,” he finished. “There’s a quote I sometimes use when I talk about this, from a French play called The Madwoman of Chaillot: ‘I know perfectly well that at this moment the whole universe is listening to us—and that every word we sa
y echoes to the remotest star.’ The capabilities of gravitational lenses make that sort of paranoia almost justified. If there’s enough capability out there to build these things, you could have a kind of ‘galactic internet,’ with everyone monitoring and talking to each other, all with very high bandwidth and very low power.”
• • •
After a half hour of outdoor ambling, we found ourselves standing before Drake’s trio of greenhouses. They were where he spent much of his time when he wasn’t caught up in his SETI work. He opened the door to the nearest one, and the hum of ventilation fans and a blast of humid, loamy air flowed out over the grass. Stepping inside, he let out a peaceful sigh. Like the other two greenhouses alongside it, this one was filled with orchids. Orchids hung from the translucent roof in pots of sphagnum moss, orchids stretched in rows on long wooden tables strewn with watering cans, and orchids sprouted from plastic buckets beneath lamps and irrigation tubes. Drake said he had about 225, but most were dormant. I counted only about a dozen blooms across the three greenhouses. He had picked up the hobby in the 1980s, about the same time he began seriously thinking about using the Sun as a gravitational lens. He did it for the challenge, he said, of nurturing the sometimes temperamental plants into full bloom, and for the satisfaction of seeing beautiful new morphological varieties emerge. Over millions of years, natural selection had shaped orchid flowers into a rich diversity of shape and color, each variety typically tuned to one or two species of pollinators. “Insects, mostly beetles,” Drake said. “They blindly shape the flowers. But the hybrids, of course, are chosen and bred by humans.”
Drake flipped on a grow lamp overhead, and in its pinkish light showed me a few blossoming hybrids, some cultivars he had cross-pollinated by hand. Each was wildly different from the others. One bore tiny flowers with trailing white petals and anthers heavy with yellow pollen. Another had five tubular, drooping purple blooms, each surrounded by a starburst of red-tinted curly leaves.
